Electron lens of magnetic field type for an electron microscope and the like

ABSTRACT

An electron lens of the magnetic field type, wherein a plurality of compensating coils are installed in a magnetic cover for a lens coil and wherein compensating currents are supplied to said compensating coils, whereby astigmatism appearing in the lens and undesired deflection of an electron beam passing through the lens are compensated by adjusting said currents.

United States Patent 1 Katagiri et al.

May 29, 1973 ELECTRON LENS OF MAGNETIC FIELD TYPE FOR AN ELECTRONMICROSCOPE AND THE LIKE Shinjiro Katagiri; Syozo Kasai, both ofHachioji, Japan Hitachi, Ltd., Tokyo, Japan Filed: May 12, 1971 Appl.No.: 142,606

Foreign Application Priority Data Japan "45/ 70630 Inventors:

Assignee:

May 13, 1970 u.s. c1. ..2s0/49.5 1)

Int. Cl ..n01 j 37/10 Field ofSearch ..250/49.5D

References Cited UNITED STATES PATENTS 3,150,258 9/l964 Welska..250/49.5D

7/ l 962 Delong Primary ExaminerJames W. Lawrence 3 Assistant ExaminerC.E. Church Attarne yCraig, Antonelli and Hill ABSTRACT An electron lensof the magnetic field type, wherein a plurality of compensating coilsare installed in a magnetic cover for a lens coil and whereincompensating currents are supplied to said compensating coils, wherebyastigmatism appearing in the lens and undesired deflection of anelectron beam passing through the lens are compensated by adjusting saidcurrents.

12 Claims, 13 Drawing Figures I be Patented May 29, 1973 3,736,423

5 Sheets-Sheet 1 FIG. lu

INVENTORI:

SHINJ'IRO MTAGIRI a 5YOZ0 KASAI ATTORNEYS Patented May 29, 1973 5Sheets-Sheet 2 INVENTORJ SHINTIRO KATAGIRIQMI SYOZO KASAI BY 0'ATTORNEYS I Patented May 29, 1973 5 Sheets-Sheet 5 FIG. 4b

INVENTORS SHINIRO KATAGIRI Glad SYOZO KASA' BY CA; a m

ATTORNEYS Patented May 29, 1973 3,736,423

5 Sheets-Sheet 4 FIG. 5a 7 A 2/ Q 345 -w Y m Vb J I V Vb COIL A COIL DcolgB INVENTOR) SHINTIROKATAGIRI and 5YOZO MSA W amt-M0252; v m

ATTO RN EYS Patented May 29, 1973 5 Sheets-Sheet 5 FIG 60 FIG. 6b

INVENTOKS smumo Km-Aum a. SYoza KASAI ATTORNEYS ELECTRON LENS OFMAGNETIC FIELD TYPE FOR AN ELECTRON MICROSCOPE AND THE LIKE BACKGROUNDOF THE INVENTION This invention relates to an electron lens of themagnetic field type for an electron microscope and the like, and moreparticularly to an electron lens having means for compensating forastigmatism and undesirable deflection of an electron beam therein.

As is well known, in an electron lens of the magnetic field type whichis commonly used in an electron microscope and the like, astigmatismoccurs on account of the asymmetry of the mechanical construction andmagnetic characteristic thereof.

Moreover, undesirable deflecting magnetic fields occur in the area ofcontacting portions between the pole pieces and magnetic path members insaid lens to deflect an electron beam therein.

Due to such astigmatism and undesirable deflection of the electron beam,the image produced by the beam is distorted, and further it isimpossible to observe said image within a constant viewing field sincethe image is shifted therefrom so that the ability of an electronmicroscope and the like using said lens to produce an acceptable imageis unsatisfactory and manipulation thereof becomes complicated.

In a conventional electronlens, compensating coils are disposed in thevacuum space of the pole piece opening or beam passage in said lens soas to eliminate said astigmatism and undesirable deflection of theelectron beam. However, such an'arrangement is undesirable since thelead lines of said compensating coils must be taken out from said vacuumspace without producing leakage and it is required to insulate betweensaid compensating coils and the magnetic circuit of said lens.

Therefore, such an arrangement requires a complicated construction toattain the above requirements so that the production costs of said lensbecomes prohibitive. Moreover, since the compensating coils are disposedin the vacuum space, the reluctance with respect to the magnetic fluxproduced by said compensating coils is very large, and thus thecompensating currents supplied to said compensating coils must beincreased.

SUMMARY OF THE INVENTION An object of this invention is to provide anelectron lens so constructed that astigmatism and undesirable BRIEFDESCRIPTION OF THE DRAWINGS FIG. la is a longitudinal section showing anembodiment of this invention;

FIGS. 1b and 1c are cross sections of respective embodiments ofcompensating coil arrangements taken along line lb, c lb, c in FIG. la;

FIG. 2 is a partial sectional view showing the location of compensatingcoils in another embodiment of this invention;

FIG. 3a is a partial sectional view showing the main part of a furtherembodiment of this invention;

FIG. 3b is a section taken along line IIIb IIIb in FIG. 3a;

FIG. 4a is a longitudinal section showing a still further embodiment ofthis embodiment;

FIG. 4b is a cross section taken along line IVb .IVb of FIG. 4a;

FIG. 5a is a longitudinal section showing the main part of a stillfurther embodiment of this invention;

FIG. 5b is a cross section taken along line Vb Vb of FIG. 5a;

FIG. 6a is a longitudinal section showing still another embodiment ofthis invention;

FIG. 6b is a cross section taken along line Vlb Vlb of FIG. 6a; and

FIG. 7 is a circuit diagram showing a circuit for supplying acompensating current to said compensating coils.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. la, an electron lensincludes an upper magnetic pole piece 1, a lower magnetic pole piece 2,an upper magnetic path member 3, a lower magnetic path member 4, anexciting lens coil 5 wound in an annular shape to surround the electronbeam passage, and a spacer 6 made of non-magnetic'material. The upperand lower magneticpole pieces 1 and 2 and the upper and lower magneticpath members 3 and 4 constitute a magnetic cover disposed to cover theexciting lens coil 5. These are well known elements which constitute aconventional electron lens structure of magnetic field type. In such anelectron lens, the spacer 6 is not always used.

In accordance with the invention, the electron lens further includes acompensating coil device 7 having four coils A to D, as shown in FIG.lb, which are wound on the lower magnetic path member 4 through holesformed therein. In this case, the respective coils are disposed to havean angle of formed between two transverse center axes of said coilsadjoining each other, as shown in FIG. lb. As also shown, thecompensating coils may be recessed in the cover body so as not toprotrude therefrom.

The coil A is connected to coil C with an opposite sense thereto inseries and the coil B is connected to coil D in the same manner.Compensating currents are supplied to said coils ,in such a manner thatmagnetic flux a, and flux da produced by said coils A and C are in thedirection of an electron beam axis E, and magnetic flux and flux (1),,produced by said coils B and D are in the opposite direction thereof soas to compensate for astigmatism and undesirable deflection of theelectron beam.

As is well known, with the above-mentioned compensating coil device,only the astigmatism appearing in a specific direction may be corrected.In order to make it possible to correct astigmatism in whicheverdirection it occurs, the compensating coil device may be constructed byeight coils-A to H, e.g., as shown in FIG. lc.

The respective coils are wound on the lower magnetic path member 4through holes formed therein as in the foregoing, and they are disposedin such am manner that the angle defined by the central axes of theadjacent two coils is 45.

The coils A, C, E and G are connected in series with the sense ofalternate coils being the same. Thus, they are energized in such amanner that, as in the above description, d), and 41 which are among themagnetic fluxes produced by the respective coils opposing each other,are directed towards the electron beam axis E; whereas, and are directedaway from the axis. Similarly, the coils B, D, F and H are connected inseries, and are energizedas described above.

As apparent from the above description, the compensating coil device inFIG. comprises in combination two sets of the one in FIG. 1b. Thecompensating current which is caused to flow through one set of thecoils,

A, C, E and G is made so as to differ in phase by 90 with respect to thecompensating current which is caused to flow through the other set ofthe coils B, D, F and IL In other words, if the current through one setof the coils is varied in accordance with the cosine of the angle, sothat the resultant magnetic field may rotate by 360. In this manner, inwhichever direction astigmatism appears, it may be corrected.

Even if eight coils are not used, astigmatism generated in any directionmay be corrected with only four coils A, A, B and B insofar as they arearranged as illustrated in FIG. 6b. In FIG. 6b, the mutually opposingcoils A and A, and B and B' are connected in series, respectively, whilethey are disposed in such a manner that the central axes of the coils ofthe set consisting of A and A and those of the coils of the setconsisting of B and B define an angle of 45, respectively.

The same result as achieved in the previous embodiment is obtained withthe exciting currents varying in accordance with the sine and cosine ofthe angle being supplied to the respective sets of the coils. Thesecoils are assembled into a magnetic cover with the pole piece and themagnetic path member formed integrally, e.g., as illustrated in FIG. 6a.

The embodiment shown in FIG. 2 is a modification of the embodiment inFIG. 1a, and has the deflecting coil embedded in the lower magnetic pathmember 4. The feature of this embodiment is that the leakage magneticfield from the magnetic cover may be reduced.

Since the deflecting coil 7 is wound into the iron core in each of theembodiments shown in FIGS. 1a and 2, the magnetic reluctance throughwhich the magnetic flux permeates is low, and the correcting effect isstrong. Therefore, even when the deflecting coil 7 is provided at thatpart of the lower magnetic path member 4 which is considerably distantfrom the electron lens gap, its effect may be sufficiently utilized.

FIGS. 3a and 3b illustrate a further embodiment with the deflecting coil7 mounted on the lower pole piece 2. In the figures, numeral 8designates a protecting tube. The feature of this embodiment resides inthat, since the compensating coil device 7 is assembled in the vicinityof the electron lens gap, the correcting effect is strong and hence itmay accomplish the object of the invention satisfactorily even in casewhere only a small amount of excitation is provided.

A still further embodiment shown in FIGS. 4a and 4b has the deflectingcoil 7 installed at that part of the magnetic path member which is at aposition as distant from the electron beam axis as possible. Althoughthe figures illustrate a structure wherein four coils A, B, C and Dconstituting the deflecting coil 7 are provided so as to enable thecorrection of incorrect deflection of an electron beam and astigmatismin a specific direction, it is also possible to construct the deflectingcoil so as to correct astigmatism in any direction by providing eightcoils, as seen in FIG. lc.

In an embodiment illustrated in FIGS. 5a and 5b, the

compensating coil device 7 is assembled in the magnetic cover in thevicinity of the electron lens gap, and yet, it is disposed so as not toappear within the vacuum space of the electron beam path.

As shown in FIG. 7 by way of example, the compensating coil device Ato'D in FIG. 1b may be connected to the lens coil 5 through resistors Rand R and variable resistors R and R thereby to supply predeterminedcurrents from an exciting power source 9 to both of the coils.

With such construction, the excitation of the respective coils varies inproportion to each other. Therefore, once the exciting currents of therespective compensating coils are adjusted by means of the variableresistors R and R so as to enable the correction of astigmatism andundesired deflection, it is unnecessary to again adjust the compensatingcurrents even if the excitation of the electron lens is changed.

While we have shown and described several embodiments in accordance withthe present invention, it is understood that the same is not limitedthereto but is susceptible of numerous changes and modifications asknown to a person skilled in the art and we therefore do not wish to belimited to the details shown and described herein but intend to coverall such changes and modifications as are obvious to one of ordinaryskill in the art.

What is claimed is:

1. In an electron lens of the magnetic field type for an electronmicroscope and the like, comprising an exciting lens coil wound in anannular shape, a magnetic cover disposed to cover said lens coil andcomposed of an upper magnetic path member, a lower magnetic path memberand upper and lower magnetic pole pieces, the improvement whichcomprises a plurality of compensating coils each being wound ondifferent portions of said magnetic cover through holes providedtherein, supply means for supplying compensating currents to saidcompensating coils and adjusting means for adjusting said compensatingcurrents whereby astigmatism and undesired deflection of an electronbeam passing through said lens can be eliminated.

2. An electron lens according to claim 1, characterized by eightcompensating coils, which are respectively arranged such that thecentral axes of adjacent coils define an angle of 45 therebetween.

3. An electron lens according to claim 1, characterized in that saidcompensating coils are embedded in said magnetic cover.

4. An electron lens according to claim 1, characterized by fourcompensating coils, the central axes of one pair of coils opposing eachother defining an angle of 7 45 with those of the other pair of coilsopposing each other.

said compensating coils are provided in said lower magnetic path member.

6. An electron lens according to claim 5, wherein said compensatingcoils are disposed symmetrically about said lower magnetic pole piece.

7. An electron lens according to claim 1, wherein said compensatingcoils are disposed around said lower magnetic pole piece.

8. An electron lens according to claim 7, wherein said upper and lowermagnetic pole pieces define a central passage and an insulating tube isdisposed in said central passage formed by said lower pole piece toprotect said compensating coils.

9. An electron lens according to claim 1, wherein said upper and lowerpole pieces form a magnetic lens gap therebetween, said compensatingcoils being disposed in said lower magnetic path member in a planesubstantially passing through said lens gap.

10. An electron lens according to claim 9, wherein four compensatingcoils are provided in said lower magnetic path member so as to occupy arespective quadrant about said lens gap.

11. An electron lens according to claim 1, wherein said compensatingcoils are wound in planes parallel to the axis of said lens coiladjacent a passage through said upper and lower pole pieces.

12. An electron lens according to claim 11, characterized by .fourcompensating coils, the central axes of one pair of coils opposing eachother defining an angle of 45 with those of the other pair of coilsopposing each other.

1. In an electron lens of the magnetic field type for an electronmicroscope and the like, comprising an exciting lens coil wound in anannular shape, a magnetic cover disposed to cover said lens coil andcomposed of an upper magnetic path member, a lower magnetic path memberand upper and lower magnetic pole pieces, the improvement whichcomprises a plurality of compensating coils each being wound ondifferent portions of said magnetic cover through holes providedtherein, supply means for supplying compensating currents to saidcompensating coils and adjusting means for adjusting said compensatingcurrents whereby astigmatism and undesired deflection of an electronbeam passing through said lens can be eliminated.
 2. An electron lensaccording to claim 1, characterized by eight compensating coils, whichare respectively arranged such that the central axes of adjacent coilsdefine an angle of 45* therebetween.
 3. An electron lens according toclaim 1, characterized in that said compensating coils are embedded insaid magnetic cover.
 4. An electron lens according to claim 1,characterized by four compensating coils, the central axes of one pairof coils opposing each other defining an angle of 45* with those of theother pair of coils opposing each other.
 5. An electron lens accordingto claim 1, wherein said compensating coils are provided in said lowermagnetic path member.
 6. An electron lens according to claim 5, whereinsaid compensating coils are disposed symmetrically about said lowermagnetic pole piece.
 7. An electron lens according to claim 1, whereinsaid compensating coils are disposed around said lower magnetic polepiece.
 8. An electron lens according to claim 7, wherein said upper andlower magnetic pole pieces define a central passage and an insulatingtube is disposed in said central passage formed by said lower pole pieceto protect said compensating coils.
 9. An electron lens according toclaim 1, wherein said upper and lower pole pieces form a magnetic lensgap therebetween, said compensating coils being disposed in said lowermagnetic path member in a plane substantially passing through said lensgap.
 10. An electron lens according to claim 9, wherein fourcompensating coils are provided in said lower magnetic path member so asto occupy a respective quadrant about said lens gap.
 11. An electronlens according to claim 1, wherein said compensating coils are wound inplanes parallel to the axis of said lens coil adjacent a passage throughsaid upper and lower pole pieces.
 12. An electron lens according toclaim 11, characterized by four compensating coils, the central axes ofone pair of coils opposing each other defining an angle of 45* withthose of the other pair of coils opposing each other.